Firefly is a science fiction television series created by Joss Whedon (who also created the "Buffy the Vampire Slayer" series about a renegade spaceship crew). It aired for three months on FOX. In 2005, the feature film Serenity was released by Universal Studios to pick up where the series left off. Several episodes of The Big Bang Theory have addressed Sheldon's fondness for the Firefly series.
In "The Large Hadron Collision" (S03E15), Sheldon starts a game of Traitors with Leonard. Round two includes Leonard, Darth Vader and Rupert Murdoch. Sheldon includes Rupert Murdoch because "he owns Fox, and they canceled Firefly."
In "The Staircase Implementation" (S03E22), a flashback shows Sheldon and Leonard working out their roommate agreement. In the "television and movies" section, Sheldon decrees that their Friday nights shall be spent watching Firefly, assuming it'll be on for years.
In "The Hawking Excitation" (S05E21), Sheldon is begging Howard to introduce him to Stephen Hawking, but according to Raj, the first time Sheldon ever begged for anything was when he begged FOX not to cancel Firefly.
In "The Comic Book Store Regeneration" (S08E15), Leonard and Raj recognize Nathan Fillion (Captain Reynolds), in a deli. They take a picture with him, although Fillion is very annoyed by Raj's actions.
- Interestingly, despite Sheldon being a huge fan of the show, he never actually interacted with any of the actors.